Treatment of alloys



Patented Oct. 9, 1934 UNITED STATES TREATMENT OF ALLOYS Joseph C. Dittmer, St. Albans, N. Y., assignor to National Lead Company, a corporation of New Jersey No Drawing.

Application September 17, 1931,

Serial No. 563,361

7 Claims.

The invention is a method of selectively eliminating or reducing the proportions of certain constituent metals in so-called white metals, and particularly in lead base alloys, its object being to change the composition of such alloy, or purify the metal, with greater facility and selectivity and a less cost for time and materials than possible by methods now generally in use. More especially it consists in the discovery I have made that caustic alkali and water vapor or steam, provided they are properly brought into contact with the molten alloy, by using any adequate means of agitation, exercise an efficient oxidizing reaction upon certain metals and can be controlled to exercise a very preferential reaction upon some of them with considerably less effect on the others than has ever been known before in the art so far as I am aware. The caustic alkali also reacts upon the oxides of the metals removed, forming their respective alkali oxy-salts which are dissolved or held in suspension in the alkali mass for subsequent recovery.

It is well known to those experienced in this art that Where, for example, it is desired to completely remove arsenic and/or tin from an antimony-lead alloy, the methods hitherto used will unavoidably remove more or less antimony and lead along with the arsenic and/or tin. Where, say, the percentages of tin in the alloy is 1% or less while that of antimony is greater, it is likely that more, or at least as much antimony as tin will be removed by the processes now generally in use; one of which is the well known treatment with molten caustic soda (with or without the addition of sodium chloride) and an oxidizing agent such as sodium nitrate. Air has also been used. as the oxidizing agent in conjunction with caustic soda but the action is slow as compared with that obtained by sodium nitrate, and the selectivity of the separation of the arsenic and/or tin from antimony and lead is not as satisfactory as could be desired.

Other processes have been proposed, for which greater selectivity is claimed but many of them have the disadvantage of being too complicated for economical application.

When using caustic soda and sodium nitrate for the removal of tin from an alloy containing, for

59 example, about 5% $11., 11% Sb, and 84% Pb, a

found that by the application of. my herein described process to the treatment of the same type of alloy of similar composition and with the proper temperature control I have been able to secure complete removal of the tin and obtain a removal ratio as high as 146 parts of tin to 1 part of antimony. This is shown by Table No. 2, below.

As an illustration of the simplicity of my process, I shall describe a method of applying it, but I do not restrict myself to this method of application. The alloy for treatment is melted in an ordinary melting pot and, in the case where it is desired to remove certain of the constituent metals without removing antimony it is held at a temperature of about 350 C., and the surface of the molten alloy is covered with an amount of molten caustic soda sufiicient to keep the metal covered during the treatment and to react with and take up the oxides of the metals to be removed by the treatment. The pot is loosely covered as with an ordinary lid or cover, and steam is admitted to the space between the surface of the molten caustic soda and the cover. The metal and caustic soda are at the same time vigorously agitated by a mechanical stirrer, so as to make a vortex in the metal thus bringing the caustic soda and steam into very intimate contact with each other and the metal. A vigorous reaction takes place accompanied by the evolution of hydrogen gas and the caustic mass thickens rapidly, indicating the absorption by the latter of the oxy-salts of the metals removed. The antimony w ll not be appreciably aifected. The completion of the treatment is indicated by the cessation of gas evolution and also, usually tosome extent by the consistency of the caustic mass.

It is well known that steam, by itself, has been. used to some extent in refining impure lead, containing as impurities small amounts of antimony, tin, arsenic, zinc, etc., but its principal purpose, where so used, has been to agitate the metal so that its surface, kept at an elevated temperature above 500 C., is exposed to the action of air. This oxidation by air aided by steam is not particularly selective because zinc, arsenic, tin, antimony and lead are all more or less oxidized at about the same time.

It is also known that zinc, arsenic, tin, antimony and lead are all more or less acted upon and oxidized to form their respective sodium salts, when treated with molten caustic soda, by itself, that is, without the use of a special oxidizing agent such as nitrate, the extent of the action depending upon the metal being acted upon and the temperature of the reaction. To produce any considerable elimination of one or more of the more easily oxidized metals, such as arsenic and tin, with caustic soda alone, it is necessary to employ a much higher temperature than is necessary or desirable in my process and this higher The use together of steam and caustic, however, provides a degree of selectivity which is not attainable by the use of either alone, and by carrying it out at temperatures near the melting point of the alloy, it is possible to remove zinc, arsenic and tin and some other metals completely without appreciable action upon the antimony, bismuth or lead. Other things being equal, the lower the temperature the sharper the selectivity. Experiments indicate that as far as concerns the primary oxidation of the constituent metals to be removed, there is a catalytic relation between the caustic and steam; either the caustic soda may be regarded as being a catalyzer for the steam or vice versa.

The evolution of hydrogen during the oxidizing reaction upon the zinc, tin or arsenic, etc. is an excellent indicator of the progress of the treatment, for when the reaction upon these metals is complete, practically no further evolution or hydrogen takes place. This also indicates that the lead or antimony and lead, as well as certain other metals if present, such as bismuth, copper and silver, are not being acted upon. This is advantageous because there is, therefore, no danger or overtreating the alloy and no close supervision is necessary. In industrial practice when reagents such as caustic soda and sodium nitrate are used for eliminating impurities from lead, or from lead and antimony, the amount of the oxidizing agent, such as sodium nitrate, required, can only be calculated approximately and frequently the alloy has to be tested during or after treatment to determine if all the tin and arsenic have been removed. This causes loss of time besides requiring close chemical control. Where the antimony contents and the tin and arsenic contents .vary considerably in a number of alloys to be treated, it is not always possible to determine, in advance, the exact amount of nitrate needed because varying proportions of the total nitrate will be consumed by the antimony, this being particularly the case when the alloy contains a comparatively high percentage of antimony and very little tin. Because caustic soda and steam together have practically no oxidizing eii'ect upon the antimony and lead at the comparatively low temperatures I prefer to use, small or large amounts of zinc, tin or arsenic can be removed with great economy.

As to the means of applying the steam, experiments have shown that it is simply necessary to have and to keep sufficient steam or water vapor present in the more or less enclosed space over the molten caustic soda, and in intimate contact with it. If the treatment is conducted in an autoclave equipped with a condenser to prevent the loss of water vapor, a minimum quantity of steam is consumed. In a pot or kettle loosely covered or partly open, it is necessary to use more steam. It is also possible to generate the steam in situ and to keep the space over the caustic alkali filled with water vapor by dropping or spraying water upon the surface of the molten alkali covering the metal. This is not so desirable, however, because it tends to cool the molten alkali and sometimes causes foaming and spattering of the alkali which may be dangerous. A more advantageous procedure although open to the same objections, is that of dropping, or spraying a water solution of caustic soda or of any other alkali or salt upon the surface or under the surface of the molten metal or molten alkali. Aside from the unpleasant spattering and mist and the loss of heat, ex-

cellent results were obtained in an experiment in which a hot 50% water solution of caustic soda was slowly dropped into the kettle where it was rapidly drawn down into the vortex produced by a stirrer and thereby dispersed into the mass of the metal below its surface. All the tin and practically no lead or antimony were removed from an alloy which originally contained 5% Sn and 11% Sb. This was done in three hours. When steam is introduced as such. it is preferably super-heated before being admitted to the pot, thus reducing the loss of heat from the molten alkali.

Another means of introducing water vapor is by the slow and continuous addition of a salt containing water of crystallization, such as, for example, hydrated sodium carbonate (Na2COa.10H2O), to the molten caustic soda covering the metal under treatment, the salt being rapidly drawn down with the caustic under the surface of the metal by the stirrer. The salt is dehydrated, giving up its water of crystallization which together with the caustic soda results in the oxidation of and elimination of certain constituent metals as stated, provided it is added in sufiicient quantity and so introduced that the contained water does not escape before it and the caustic soda are drawn down into intimate contact with the metal. Any other hydrated salt, compound or deliquescent material innocuous to the materials under treatment may also serve as the vehicle for the water, although this method is not preferred, steam being of course cheaper.

The caustic soda which I prefer to use, may 110 be an ordinary technical grade of granular caustic soda which contains small amounts of sodium carbonate and sodium chloride. Experiments have shown that additional amounts of sodium carbonate and/or of sodium chloride 115 may be used with the caustic soda, but there is no advantage or particular disadvantage in adding these reagents. Caustic Potash (KOH) may replace part of the caustic soda required but this more expensive reagent is not essential for 120 obtaining the purification or the selectivity of separation desired.

The temperature of treatment is important principally for the sake of greater economy, control and selectivity and for lower costs of equip- 125 ment maintenance. As the temperature of treatment is increased above the preferred optimum temperature which is about 350 C., the selectivity decreases and the cost increases.

Where my process is used in conjunction with the treatment by caustic soda and sodium nitrate, in order to gain the above advantages to a greater extent than possible by means of the latter treatment alone, the temperature is a more important factor because as it increases above about 375 C., the selectivity of the separation, for example, of tin from the antimony in the alloy, rapidly decreases from the ratio of well over 50 to 1, until, at about 425 C., it is about the same (3 to 1) as that ordinarily obtained by the treatment with caustic alkali and nitrate without the use of intermixed steam.

It is generally preferable to use a temperature near 350 C., because while greater selectivity is obtained at temperatures below this, lower temperatures are generally difiicult to maintain as they may be too near to the solidification point of the alloy.

Whereas any means of agitation may be used, it will be understood that the process depends upon the action of the caustic and steam conjointly, that is to say, the mutual action of these two agents with each other as well as with the alloy. Accordingly such agitating means as is employed must be capable of producing a. rapid intimate contact of the alkali and steam simultaneously with each other and with the metal being treated equal to that produced by the vortex method. Stirring so as to produce a vortex which draws in the alkali and steam together is believed to be the best way but I do not limit myself to this method if equal simultaneous contact of all three materials can be produced otherwise.

The results possible of attainment by my method of treatment are indicated by the following tables:-

Table No. 1

Materials used:

8948 grams of original alloy. 2000 grams of caustic soda (750 g. being sufficient). Steam. Products obtained:

8786 grams of treated alloy.

Alkali salts.

Original Treated Analysis 0y Alloy Percent Percent Lead 91. 70 93. 42 2. 84 2. 89 3. 61 3. 65 04 .04 l. 78 None. Ar anio .03 None.

Removals of... Lead Bis- Anti- Copper 'Iin Arsenic muth mony 3 g. None. 2 g. None. 159 g. 3 g.

Removal ratio of:

Tin to antimony=79 to 1.

Table No. 2 Materials used:

200 lbs. of original alloy. 45.84 lbs. caustic soda. Steam. Products obtained:

189 lbs.5 oz. (85,872 grams) of treated Removal ratio of:

Tin to antimony=146 to 1.

The time of the above treatment, using steam power stirrer, was about three hours.

The new method is economical not only because of the selective separation possible between the two classes of metals (i. e. those acted upon and those not or little acted upon), but because of the shortened time of treatment, lower temperature and heating costs, lower cost of steam as compared to the use of sodium nitrate or other oxidizing agents, less supervising, less chemical control and less production of salts of the lead, antimony and other metals, the removal of which is not desired and which salts would have to be converted back into metal or otherwise salvaged. The method is principally applicable to separation of tin and arsenic from lead base alloys but may be used for removing certain other metals from lead base alloys and also from any one, or more, of the metals not appreciably acted upon. Generally speaking, in addition to zinc, tin and arsenic, the metals of the alkali and alkali earth groups as well as aluminum, silicon, selenium, and tellurium, are subject to preferential oxidation by this method, as compared with such alloying metals as lead, antimony, bismuth, and copper which are found not to be acted upon to any appreciable extent within the range of temperature referred to. At higher temperatures, the reaction still takes place, but it is less selective.

I claim:

1. Method of removing from lead or lead alloys of antimony, bismuth or copper, any one or more of the metals in a group containing tin, arsenic, aluminum, silicon, selenium, tellurium, alkali metal, or alkali earth metals, which consists in assembling a body of the molten alloy with bodies of molten caustic alkali and water vapor, intimately contacting these three bodies with that degree of intimacy as is obtained by allowing the molten caustic and steam to be drawn down into a whirling vortex of the metal, thereby producing simultaneous and continuous contact of each with both the others notwithstanding their difference of specific gravity, and while at a temperature of approximately 425 C. or less.

2. Method of removing from lead or lead alloys of antimony, bismuth or copper, any one or more of the metals in a group containing tin, arsenic, aluminum silicon, selenium, tellurium, alkali metal, or alkali earth metals, which consists in vertically stirring the molten alloy by mechanical means and causing it to draw down into the vortax simultaneously water vapor and caustic alkali, thereby producing conjoint action thereof upon said alloy and maintaining the alloy at a temperature below the point where substantial oxidation occurs of the metal or metals not desired to be removed.

3. Method of removing tin or arsenic from lead alloys containing antimony which consists in reacting on the molten alloy conjointly with molten caustic alkali and water vapor by contacting such materials each with the other with that degree of thoroughness as is obtained by vortically stirring by mechanical means and allowing the caustic and steam to be drawn down into a whirling vortex of the metal, at a temperature at which the antimony is not substantially affected.

4. Method of removing tinfrom lead alloys containing antimony which consists in reacting on the molten alloy with molten caustic alkali and water vapor, by bringing all three of these materials into simultaneous continuous contact, each with both the others, at a temperature at which the antimony is not oxidized in excess of a ratio of 1 to 3 as compared to the oxidation of the tin.

5. Method of removing tin from dead alloys containing antimony which consists in assembling with the molten alloy a body of molten caustic alkali and water vapor, vortically stirring .the melt to cause the alkali and water in mutual contact to be drawn down into the vortex for conjoint and simultaneous reaction on the alloy while maintaining the melt at a temperature of not in excess of approximately 350 C.

6. Method of removing from lead or lead alloys oi" antimony, bismuth or copper, any one or more 01' the metals in a group containing tin, arsenic, aluminum, silicon, selenium, tellurium, alkali metals, or alkali earth metals, which consists in assembling a body of the molten alloy with bodies of molten caustic alkali and water vapor, inti-' mately contacting these three bodies with that degree of intimacy as is obtained by vortically stirring the melt by mechanical means and allowing the molten caustic and steam to be drawn down into a whirling vortexof the metal, thereby producing simultaneous and continuous contact.

of each with both the others notwithstanding their difierence of specific gravity, and while at a temperature at which the antimony, lead or bismuth is not oxidized in excess of a ratio of 1 to 3 ascompared to the oxidation of the constituent or constituents to be removed.

7. Method of removing from lead or lead alloys of antimony, bismuth or copper any one or more of the metals in a group containing tin, arsenic, aluminum, silicon, selenium, tellurium, alkali metal or alkali earth metals, which consists in assembling a body of the molten alloy with bodies of molten caustic alkali and water vapor, vortically stirring the molten alloy by mechanical means and causing it to draw down in the vortex simultaneously water vapor and caustic alkali, thereby producing simultaneous and continuous contact of each with both the others notwithstanding their difference of specific gravity and in the meantime maintaining the alloy at a temperature at which the lead, antimony, bismuth or copper is not oxidized in excess of a ratio of one to three as compared to the oxidation of the metal or metals to be removed.

JOSEPH c. DITTMER. 

